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CHM210H1 (2)
Lecture 3

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University of Toronto St. George

Lecture 3 Reading – Pages 20-33 There are additional processes that work to destroy ozone in the stratosphere, in addition to the ones described in the Chapman mechanism - These all involve catalysts that are present in air, like chlorine and bromine Mechanism I – a number of atomic and molecular species, designated as X, are able to react efficiently with ozone by abstracting (removing) an O atom from it - X + O  XO + O 2 2 The XO molecules react with O atoms to produce O and refo2m X - XO + O  X + O 2 Overall reaction corresponding to this reaction mechanism – obtained by adding the steps that occur in air repeatedly an equal number of times - In this case, the reactants in the 2 steps are added together and become the reactants of the overall reaction; same case for the products - X + O +3XO + O  XO + O + X + O 2 2 - Molecules that appear on both sides of the reaction equation are cancelled out, leaving the balanced overall reaction O + 3  2O 2 Species X are catalysts for ozone destruction in the stratosphere, since they speed up a reaction, but are eventually reformed intact, and can start the cycle again X catalysts greatly increase the efficiency of the reaction and decrease the steady-state concentration of ozone - We have inadvertently increased the stratospheric concentrations of such X catalysts by releasing certain gases (esp. those containing Cl and Br) at ground level; this has contributed to all the environmental concerns about ozone depletion Most ozone destruction by Mechanism I occurs in the middle/upper stratosphere, which has an already low ozone concentration Chemically, all X catalysts are free radicals – atoms/molecules containing an odd number of electrons - Free radicals are very reactive, due to a driving force for their unpaired electron to pair with one of the opposite spin, even if it’s located in a different molecule Rate of a chemical reaction is influenced by parameters such as the magnitude of the activation energy required for the reaction to occur In gas-phase reactions involving free radicals as reactants, the activation energy exceeds that imposed by their endothermicity by only a small amount - So we can assume that all exothermic free-radical reactions will have a small activation energy, and occur very quickly Small amounts of the X catalysts have always been present in the stratosphere, so the catalytic destruction of ozone can occur even in a “clean” (unpolluted) atmosphere Nitric oxide (NO) – free-radical molecule responsible for catalytic ozone destruction - Formed when molecules of nitrous oxide (N O) rise2from the troposphere to the stratosphere; they could eventually collide with an excited O atom produced by photochemical decomposition of ozone - N 2 + O*  2NO Nitrogen dioxide (NO ) – 2ormed when the NO molecules catalytically destroy ozone by extracting an O atom from ozone - NO + O  NO + O 3 2 2 - NO +2O  NO + O 2 - O 3 O  2O 2 overall reaction Atomic oxygen has to complete the cycle of Mechanism I by reacting with XO for the X catalyst to be regenerated in a usable form - XO + O  X + O 2 Mechanism II – destruction of ozone without atomic oxygen; depletes ozone in the lower stratosphere, esp. when the concentrations of X catalysts are high - Accounts for the majority of ozone depletion by man-made chemicals, esp. in ozone holes 2 ozone molecules are destroyed by the same catalysts, and by the same initial reaction - X + O 3XO + O 2 - X’ + O  X’O + O 3 2 nd - X’ indicates that the 2 catalyst doesn’t have to be chemically identical to X o One of X or X’ must be a Cl atom; the other one can be Cl or Br - When XO and X’O react with each other (they have an added oxygen atom), X and X’ are regenerated o Before X and X’ are regenerated in a usable form, the combined but unstable molecule XOOX’ is formed, then decomposed by heat or light o XO + X’O  [XOOX’]  X + X’ + O 2
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